19th November 2013: Steering electrons along chemical bonds Electron motions induced by a strong electric field are mapped in space and time with the help of femtosecond x-ray pulses. An x-ray movie of the crystal lithium hydride shows that the electric interaction between electrons has a decisive influence on the direction in which they move. ... more.
15th November 2013: Confinement rules electrons’ race Ultrashort flashes of light with just the right energy can shine on an atom confined in a fullerene cage in order to knock electrons out of various quantum levels. In a recent theoretical prediction, it is found that such a confinement shows a spectacular preference: it lets one electron to escape faster than the other. For free atoms, the delayed response of escaping electrons to light is known. ... more.
18th July 2013: Attosecond electron dynamics in molecules One of the visions guiding current research on the application of attosecond (1 as = 10-18s) laser pulses to molecules is the possibility of controlling molecular processes by manipulating valence electrons on attosecond to few femtosecond (1 fs = 10-15s) timescales. As predicted by several theoretical works in the last decade, the ultrafast removal (on attosecond timescales) of electrons from molecules may lead to ultrafast migration of the hole in the charge density along the molecular skeleton, potentially influencing the subsequent chemistry of the molecule in what is sometimes called “charge-directed reactivity” ... more.
15th July 2013: A Million Times Brighter than the Sun - White Light as an Extremely Short Pulse Researchers from the Leibniz Universität Hannover, the Weierstraß Institute for Applied Analysis and Stochastics as well as the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy have developed a new concept to generate optical white-light pulses in the visible and near infrared. These pulses can be compressed to extremely short pulse duration, comprising only a single oscillation of the optical carrier field.... more.
21st May 2013: Hydrogen atoms under the magnifying glass: Direct Observation of the Nodal Structures of Electronic States of the Hydrogen Atom To describe the microscopic properties of matter and its interaction with the external world, quantum mechanics uses wave functions, whose structure and time dependence is governed by the Schrödinger equation. In atoms, electronic wave functions describe - among other things - charge distributions existing on length-scales that are many orders of magnitude removed from our daily experience. ... more.
15th April 2013: Neighbors move electrons jointly - an ultrafast molecular movie on metal complexes in a crystal Applying femtosecond x-ray methods, researchers at the Max-Born-Institute in Berlin (Germany) and the Ecole Polytechnique Federale de Lausanne (Switzerland) observed an extremely fast, collective electron transfer of ~100 molecular ions after excitation of a single electron in a crystal of transition metal complexes. ... more.
More detailed Information:
Steering electrons along chemical bonds
Electron motions induced by a strong electric field are mapped in space and time with the help of femtosecond x-ray pulses. An x-ray movie of the crystal lithium hydride shows that the electric interaction between electrons has a decisive influence on the direction in which they move.
19th November 2013
An ionic crystal is a regular arrangement of positively and negatively charged ions in space. A prototype material is the rock salt crystal sodium chloride (NaCl) with elementary units in form of a cube. This cube contains positively charged Na+ ions in which one electron is lacking, and negatively charged Cl- ions with one extra electron (Fig. 1). Another material with this cubic structure is lithium hydride (LiH), consisting of lithium (Li) and hydrogen (H) atoms. In contrast to the ionic rock salt Na+Cl-, counting the charges in LiH gives Li0.5+H0.5-, striking a happy medium between the ionic case Li+H- and the so-called covalent case Li0+H0- in which electrons are shared between lithium and hydrogen. The peculiar behavior of LiH originates from the strong electric forces between all charged particles in the crystal. Electric interactions lead to a spatial arrangement of electrons in which the total electric energy is minimized. Application of an external electric field to the crystal sets the electrons in motion, again strongly influenced by the spatial correlations among all electrons. Electron correlations have been a subject of many theoretical studies while direct experimental insight is mostly lacking.
The team of project 3.3 has now addressed electron correlations by following ultrafast electron motions in space and time, in this way generating ‘maps’ of the electron distribution. In the experiments, electrons are set in motion by a very strong electric field which is provided for the very short time interval of 50 fs (1 fs = 10-15 s) by a strong optical pulse interacting with the LiH material. Then, a 100 fs long x-ray pulse is scattered from the ‘excited’ crystal and images the momentary electron distribution. In the current issue of Physical Review Letters http://prl.aps.org/abstract/PRL/v111/i21/e217401 [111, 217401 (2013)], Vincent Juvé, Marcel Holtz, Flavio Zamponi, Michael Woerner, Thomas Elsaesser, and Andreas Borgschulte present transient electron distributions, showing an extremely fast shift of electronic charge from the Li0.5+ to the H0.5- ions over a distance of 0.2 nm. This totally unexpected result means that the material becomes more ionic upon application of the external field, a behavior in contrast to other ionic materials such as LiBH4 or NaBH4. Since the electric field of the optical pulse reverses its direction every 1.3 fs, the electron is driven forth and back between the two sites with an extremely high speed of approximately one percent of the speed of light (c = 300.000 km/s). Immediately after the optical pulse the electrons return and the original electron distribution is restored. A qualitative explanation of the unexpected electron shift is as follows: The electric field accelerates the electrons in such a way that they are more uniformly distributed over the unit cell. Li has initially more electrons with the consequence of a loss of electrons during the optical pulse. Because of the small electron number in LiH, all electrons contribute to this effect, making the electron distribution very sensitive to correlation effects. This picture is supported by theoretical calculations of the electron distribution. The manipulation of electron distributions by strong electric fields provides control over the material’s electric properties on an extremely short time scale, a fact that may lead to applications in ultrafast electrical switches.
Fig. 1: Crystals with rock salt structure. Upper crystal: sodium chloride (NaCl) with blue balls for Na+ ions and green balls for Cl- ions. Lower crystal: lithium hydride (LiH) with small blue balls for Li0.5+ ions and white balls for H0.5- ions. The grey-shaded plane indicates the sectional views shown in Fig.2.
Figure 1 (click to enlarge)
Fig. 2 Electron distribution of LiH in the grey shaded plane of Fig. 1 for times before (left panel), during (middle), and after (right) interaction of the LiH crystal with the strong electric field of the optical laser pulse. The contours show the electron density (charge per volume). The electric field moves electronic charge from the Li0.5+ to the H0.5- ion thereby making the material more ionic.
Ultrashort flashes of light with just the right energy can shine on an atom confined in a fullerene cage in order to knock electrons out of various quantum levels. In a recent theoretical prediction, it is found that such a confinement shows a spectacular preference: it lets one electron to escape faster than the other. For free atoms, the delayed response of escaping electrons to light is known. But, is this behavior also true when the atom is taken hostage inside a fullerene? Recently, an attempt has been made to answer this by implanting an argon atom inside C60.
15th November 2013
When light with enough energy impinges on matter, light transfers the energy to the matter and subsequently electrons from various quantum levels are kicked out from the matter. However, to see when matter is absorbing light was not possible until recently, i.e., whether the response time to the absorption of light is finite leading to a time delay in photoemission, or the ejection of the photoelectron occurs instantaneously. With the tremendous advancement in technology for producing ultrashort flashes of light with attosecond duration (1 as = 10-18 s), it becomes possible to investigate several longstanding fundamental questions about the dynamical aspects of photoemission processes in real-time. A finite time delay between photoemission processes corresponding to different quantum levels in free atoms has been measured experimentally. The measured time delay varies as a function of impinged energy and could be several tens of attoseconds. A ubiquitous understanding in all these measurements is the dominant influence
of electron correlations to determine the time behavior of ejected electrons. Therefore, the reasons for the finite time delay are attributed to the complex electron-electron interactions. To model such complex electron interactions and estimate the finite time delay during a photoemission process is very challenging. Therefore, a series of provocative experiments and theoretical studies have brought the question of finite time delay in the photoemission process to the forefront.
In a study that now was being published at Physical Review Letters [111, 203003 (2013)] http://link.aps.org/doi/10.1103/PhysRevLett.111.203003 an international team of theoretician from Qatar, USA and the Max Born Institute, have attempted to settle the dust on the finite time delay controversy in free argon atom (see Figure 1), in the work led by Gopal Dixit. The time-dependent density functional method is employed to estimate the time delay between 3s and 3p quantum levels of argon atoms. The estimated time delay is connected to the energy derivative of the quantum phase of complex photoionization amplitude. After establishing the importance of electron correlations during the estimation of time delay in a free atom, it is therefore of a spontaneous interest to extend the study to test the effect of correlations on the temporal photo-response of atoms in material confinements.
A brilliant natural laboratory for such is an atom endohedrally captured in a fullerene cage (see Figure 2) that envisions the process. There are two compelling reasons for this choice: (i) such materials are highly stable, have low-cost sustenance at the room temperature and are enjoying a rapid improvement in their synthesis techniques; and (ii) effects of correlations of the central atom with the cage electrons have been predicted to spectacularly influence the atomic valence photoionization. The space and energy proximity of the argon outer electron to a C60 electron metamorphose both into two Ar-C60 hybrid electrons. Significant ground state hybridization of Ar 3p is found to occur with the C60 3p orbital, resulting in symmetric and anti-symmetric wavefunction mixing. These overlaps are critical, since a symmetric wavefunction has a structure completely opposite to that of anti-symmetric wavefunctions over the C60 shell region where each of them strongly overlaps with a host of C60 wavefunctions to build correlations. These opposing modes of overlap from one hybrid to another flips the phase modification-direction between two hybrid 3p emissions around respective Cooper minimum. As a result the peak delay of the anti-symmetric electron is approximately double to the peak advancement (negative delay) of the symmetric electron. Due to correlations with the fullerene's electrons, one hybrid escapes faster than the other by approximately 100 attoseconds upon being illuminated by the flashes of light (see Figure 3). The source of such an intriguing behavior lies in the preservation of their quantum phase. An analogy can be drawn with the conservation of the linear momentum in a two-body collision but in the time domain. The research attempts to connect two seemingly distinct disciplines of contemporary science, nanotechnology and attoscience, and motivates towards building ultrafast light sensors, where the response time of light to nanomaterials is in the attosecond domain.
Figure 1: The relative time delay between 3s and 3p quantum levels of free argon atom and its comparison with the experimental measurements at three experimental energies. Since all the ejected photoelectrons are collected in the detector, the net 3p delay must be a statistical combination, that is, the sum of the delays weighted by the photochannelâ€™s individual cross section branching ratios. As illustrated upon including 3p→ks along with 3p→kd (purple curve) this way, the shape of the relative delay strikingly alters near 3p Cooper minimum.
Figure 1 (click to enlarge)
Figure 2: Schematics of probing the effect of confinement and electron correlations on the relative time delay between the 3s and 3p photoemissions of Argon confined endohedrally in C60.
Figure 2 (click to enlarge)
Figure 3: The relative delay between 3s and hybrid 3p quantum levels (symmetric, black curve; and anti-symmetric, red curve) including the s-wave contributions and its comparison with the relative delay in free argon atom (purple curve).
One of the visions guiding current research on the application of attosecond (1 as = 10-18s) laser pulses to molecules is the possibility of controlling molecular processes by manipulating valence electrons on attosecond to few femtosecond (1 fs = 10-15s) timescales. As predicted by several theoretical works in the last decade, the ultrafast removal (on attosecond timescales) of electrons from molecules may lead to ultrafast migration of the hole in the charge density along the molecular skeleton, potentially influencing the subsequent chemistry of the molecule in what is sometimes called “charge-directed reactivity”. An important and necessary challenge that needs to be overcome in order to observe charge migration is to prove that attosecond pulses can observe this process. In a study that was published in Physical Review Letters, experimental results are published that were obtained in the new attosecond laboratories at the Max-Born-Institute and that provide the first evidence for this.
18th July 2013
Attosecond laser pulses are the shortest laser pulses that scientists are able to produce. Using a process called high-order harmonic generation (HHG), it has become possible to produce pulses with a duration of 50-500 as. In HHG, an atomic or molecular gas is typically exposed to a near-infrared driving laser pulse that is intense enough to ionize the gas. The electrons that are thereby set free experience the oscillatory electric field of the driving laser, which pulls them away from their parent ion, and then, a short time later (when the laser electric field has changed sign), accelerates them back towards this ion. In the course of the electron-ion re-collision that then occurs, the electron may be re-absorbed and all the energy that has been invested in the ionization and laser acceleration of the electron may come available via the formation of high energy photons, with an energy that is typically in the 10-100 eV range. Since all electrons that participate in the process are set free and recombine more or less simultaneously, these bursts of high energy photons have a duration that is merely a small fraction of the duration of one optical cycle of the driving laser. Since this duration is just a few femtoseconds for a near-infrared driving laser pulse, the formation of attosecond pulses is inevitable.
Attosecond pulses are a wonderful tool for studying the time-dependent motion of electrons. Whereas the motion of atomic centers is most appropriately studied with femtosecond laser pulses, making movies of electronic motion requires a time resolution in the attosecond domain. Therefore, since their discovery in 2001, many scientists have turned their attention to the application of attosecond pulses to studies of electron dynamics in atomic, molecular and condensed phase systems. In previous studies by the MBI-team on the application of attosecond pump-probe spectroscopy to a molecular system, first evidence was obtained for coupling of electronic and structural dynamics on attosecond to few-femtosecond timescales, as well as for entanglement of multiple electrons. However, the use of attosecond pulses for observing purely electronic motion in a neutral molecule thus far remained elusive.
The recently published successful observation of electron dynamics in neutral molecules, which were performed in the framework of an international collaboration with researchers from the universities of Lyon (France) and Lund (Sweden), bases itself on the technique of “dynamic alignment” that the MBI-team pioneered a little over a decade ago, and that has become a preparation step for many experiments addressing the structure and time-dependent dynamics in molecules. In a dynamic alignment experiment, a molecule is exposed to a laser pulse that is too weak to ionize the molecule, but strong enough to induce a dipole in the molecule. The interaction of the induced dipole with the laser electric field leads to a situation where the molecules align their most polarizable axis along the laser polarization axis. This allows researchers to determine how processes that are observed in the laboratory relate to processes that occur in the molecular frame.
In these recent experiments, the oscillating dipole that causes the “dynamic alignment” effect was directly observed, by ionizing the molecule with a train of attosecond laser pulses that was synchronized to the field oscillations of a near-infrared laser that induced the dipole. For a series of molecules, it was observed that the ionization probability by the attosecond pulses was markedly different at times when the oscillatory electric field of the near-infrared laser induced a dipole (Figure 1 (b)) and at times where this dipole was momentarily zero (Figure 1 (a)). The differences in the electron density in these two cases are shown for the case of molecular nitrogen in Figure 1 (c). The reason for the different ionization probabilities is that the absorption of light requires that both energy and momentum need to be conserved, in the case of the high energy photons in the attosecond pulse strongly favoring times when the electron that is to be removed is relatively close to one of the atomic centers.
The experiment was performed for a series of molecules (Figure 2), and showed that the observed effect scaled nearly linearly with the polarizability of the molecule under investigation. As described in the publication, the experiment may be viewed as an attosecond timescale implementation of the technique of molecular Stark spectroscopy, where the dependence of molecular photoabsorption on the presence of an external electric field is investigated. In the future, the team will try to measure the absorption spectrum with attosecond time resolution and will try to use the attosecond pulses to observe charge redistribution that is not driven by an external laser, but that is intrinsic to the molecule.
Synchronization of the near-infrared (NIR) field (red line) with the attosecond pulse train (APT, blue line). (a) The APT is synchronized to the zero crossings of the NIR field, while in (b) it is synchronized to the NIR field extrema. In this situation the electron density is different (compared to the undisturbed molecule) when the APT ionizes the molecule. The change of the electron density in the case of molecular nitrogen is plotted in (c). The red color denotes sites of increased density while blue shows the opposite.
The observed variation of the parent ion yield as a function of pulse delay. The molecules (molecular nitrogen, carbon dioxide and ethylene) were under the influence of a NIR field when ionized by an APT. The modulation increases with increasing polarizibility. The result of Fourier filtering around the double frequency of the NIR field is indicated by the red line.
A Million Times Brighter than the Sun - White Light as an Extremely Short Pulse
Researchers from the Leibniz University Hannover, the Weierstraß Institute for Applied Analysis and Stochastics as well as the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy have developed a new concept to generate optical white-light pulses in the visible and near infrared. These pulses can be compressed to extremely short pulse duration, comprising only a single oscillation of the optical carrier field.
15th July 2013
The generation of broadband whitelight supercontinua from spectrally narrow input laser light is one of the most fascinating processes in Nonlinear Optics and found many applications in spectroscopy, metrology and biomedical imaging. Despite of rather modest average output powers, the source area of fiber-based whitelight sources is extremely small, and they exhibit excellent spatial coherence properties like a laser. These favorable properties make fiber-based supercontinua a million times brighter than the sun.
However, the spectral coherence of such white-light sources is much poorer than the spatial one. While the light can be spatially refocused into a small spot, a similar concentration of energy along the temporal coordinate is typically impossible. In the current issue of Physical Review Letters [PRL 110, 233901 (2013)], Ayhan Demircan, Shalva Amiranashvili, Carsten Brée, and Günter Steinmeyer propose a novel method to circumvent this limiting problem of white-light sources. Rather than only launching light at one narrow wavelength region into the fiber, they demonstrate that a carefully selected combination of two pulses at well-separated wavelengths gives rise to fully compressible white light generation inside the same type of nonlinear optical fiber. The trick is to launch both pulses at nearly identical velocities, like a surfer catching a wave, to the end of inescapably locking both components together. The nonlinear interaction within this two-color optical pulse then quickly fills the spectral gap in between with newly generated coherent radiation, giving rise to coherent white light generation that can be recompressed into a short pulse.
The simulations indicate that it should be possible to directly generate optical pulses that only comprise two optical cycles of the carrier wave. Additionally, using linear dispersion compensation, even synthesis of single-cycle optical pulse should be possible. In contrast to previous methods, the resulting spectra are relatively smooth, which opens new applications, e.g., for biomedical imaging. In conclusion, the novel two-pulse white-light generation enables a wealth of previously impossible applications for these fascinating bright light sources.
Fig. 1: Numerical simulations of two-color excitation of white-light generation in a fiber. Two pulses, namely a soliton in the anomalous dispersion region and a dispersive wave in the normal dispersion region are launched at a slight delay into a fiber. While the soliton initially maintains its pulse shape, the dispersive wave quickly spreads out in time. Once both pulses start to interact, both pulse remain locked to each other and start to heavily interact with each other. Only a marginal part of the dispersive way can actually penetrate the barrier imposed by the soliton.
Direct Observation of the Nodal Structures of Electronic States of the Hydrogen Atom
21st May 2013
To describe the microscopic properties of matter and its interaction
with the external world, quantum mechanics uses wave functions, whose
structure and time dependence is governed by the Schrödinger equation.
In atoms, electronic wave functions describe - among other things - charge
distributions existing on length-scales that are many orders of magnitude
removed from our daily experience. In physics laboratories, experimental
observations of charge distributions are usually precluded by the fact
that the process of taking a measurement changes a wave function and selects
one of its many possible realizations. For this reason, physicists usually
know the shape of charge distributions through calculations that are shown
in textbooks. That is to say, until now. An international team coordinated
by researchers from the Max Born Institute has succeeded in building a
microscope that allows magnifying the wave function of excited electronic
states of the hydrogen atom by a factor of more than twenty-thousand,
leading to a situation where the nodal structure of these electronic states
can be visualized on a two-dimensional detector. The results were published
in Physical Review Letters (http://physics.aps.org/articles/v6/58)
and on physicsworld.com and provide the realization of an idea proposed approximately three decades
The development of quantum mechanics in the early part of the last century
has had a profound influence on the way that scientists understand the
world. Quantum mechanics extended the existing worldview based on classical,
Newtonian mechanics by providing an alternative description of the micro-scale
world, containing numerous elements that cannot be classically intuited,
such as wave-particle duality, the importance of interference and entanglement,
the Heisenberg uncertainty principle and the Pauli exclusion principle.
Central to quantum mechanics is the concept of a wave function that satisfies
the time-dependent Schrödinger equation. According to the Copenhagen interpretation,
the wave function describes the probability of observing the outcome of
measurements that are performed on a quantum mechanical system, such as
measurements of the energy of the system or the position or momenta of
its constituents. This allows reconciling the occurrence of non-classical
phenomena on the micro-scale with manifestations and observations made
on the macro-scale, which correspond to viewing one or more of countless
realizations allowed for by the wave function.
Despite the overwhelming impact on modern electronics and photonics, grasping
quantum mechanics and the many possibilities that it describes continues
to be intellectually challenging, and has over the years motivated numerous
experiments illustrating the intriguing predictions contained in the theory.
For example, the 2012 Nobel Prize in Physics was awarded to Haroche and
Wineland for their work on the measurement and control of individual quantum
systems in quantum non-demolition experiments, paving the way to more
accurate optical clocks and, potentially, the future realization of quantum
computers. Using short laser pulses, experiments have been performed illustrating
how coherent superpositions of quantum mechanical stationary states describe
electrons that move on periodic orbits around nuclei. The wave function
of each of these electronic stationary states is a standing wave, with
a nodal pattern that reflects the quantum numbers of the state. The observation
of such nodal patterns has included the use of scanning tunneling methods
on surfaces and recent laser ionization experiments, where electrons were
pulled out of and driven back towards their parent atoms and molecules
by using an intense laser field, leading to the production of light in
the extreme ultra-violet wavelength region that encoded the initial wave
function of the atom or molecule at rest.
About thirty years ago, Russian theoreticians proposed an alternative
experimental method for measuring properties of wave functions. They suggested
that experiments ought to be performed studying laser ionization of atomic
hydrogen in a static electric field. They predicted that projecting the
electrons onto a two-dimensional detector placed perpendicularly to the
static electric field would allow the experimental measurement of interference
patterns directly reflecting the nodal structure of the electronic wave
function. The fact that this is so, is due to the special status of hydrogen
as nature‘s only single-electron atom. Due to this circumstance, the hydrogen
wave functions can be written as the product of two wave functions that
describe how the wave function changes as a function of two, so-called
“parabolic coordinates”, which are linear combinations of the distance
of the electron from the H+ nucleus “r”, and the displacement of the electron
along the electric field axis “z”. Importantly, the shape of the two parabolic
wave functions is independent of the strength of the static electric field,
and therefore stays the same as the electron travels (over a distance
of about half a meter, in our experimental realization!!) from the place
where the ionization takes place to the two-dimensional detector.
To turn this appealing idea into experimental reality was by no means simple.
Since hydrogen atoms do not exist as a chemically stable species, they
first had to be produced by laser dissociation of a suitable precursor
molecule (hydrogen di-sulfide). Next, the hydrogen atoms had to be optically
excited to the electronic states of interest, requiring another two, precisely
tunable laser sources. Finally, once this optical excitation had launched
the electrons, a delicate electrostatic lens was needed to magnify the
physical dimensions of the wave function to millimeter-scale dimensions
where they could be observed with the naked eye on a two-dimensional image
intensifier and recorded with a camera system. The main result is shown
in the figure below. This figure shows raw camera data for four measurements,
where the hydrogen atoms were excited to states with 0, 1, 2 and 3 nodes
in the wave function for the ξ = r+z parabolic coordinate. As the experimentally
measured projections on the two-dimensional detector show, the nodes can
be easily recognized in the measurement. As this point, the experimental
arrangement served as a microscope, allowing us to look deep inside the
hydrogen atom, with a magnification of approximately a factor twenty-thousand.
Besides validating an idea that was theoretically proposed more than 30
years ago, our experiment provides a beautiful demonstration of the intricacies
of quantum mechanics, as well as a fruitful playground for further research,
where fundamental implications of quantum mechanics can be further explored,
including for example situations where the hydrogen atoms are exposed
at the same time to both electric and magnetic fields. The simplest atom
in nature still has a lot of exciting physics to offer!
Figure: (left) two-dimensional projection of electrons
resulting from excitation of hydrogen atoms to four electronic states
labeled with a set of quantum numbers (n1,n2,m) and having (from top to
bottom) 0, 1, 2 and 3 nodes in the wave function for the ξ = r+z parabolic
coordinate; (right) comparison of the experimentally measured radial distributions
(solid lines) with results from quantum mechanical calculations (dashed
lines), illustrating that the experiment has measured the nodal structure
of the quantum mechanical wave function.
Neighbors move electrons jointly - an ultrafast molecular movie on metal complexes in a crystal
Applying femtosecond x-ray methods, researchers at the Max-Born-Institute in Berlin (Germany) and the Ecole Polytechnique Federale de Lausanne (Switzerland) observed an extremely fast, collective electron transfer of ~100 molecular ions after excitation of a single electron in a crystal of transition metal complexes.
15th April 2013
Photochemistry and molecular photovoltaics make frequent use of so-called transition metal complexes which consist of a central metal ion bonded to a group of surrounding ligands. Such materials display a strong absorption of ultraviolet or visible light, making them attractive as primary light absorbers in molecular solar cells and other devices of molecular optoelectronics. Absorption of light is followed by an extremely fast shift of electrons from the metal ion to the ligands, a mechanism that is essential for generating an electric voltage. All applications rely on solid state materials in which transition metal complexes are densely packed and can interact with each other. So far, the influence of this interaction on the very fast electron motions following the absorption of light has remained unclear.
To observe ultrafast electron motions in space and time, one needs to measure the position of electrons in the material with a precision of the order of 0.1 nm (0.1 nm =10-10 m), roughly corresponding to the distance between neighboring atoms, and on a sub-100 fs time scale (1 fs = 10-15s). This is possible by imaging the material with extremely short x-ray pulses which are scattered from the electrons and provide their spatial arrangement. The electron motions are initiated by an ultrashort optical pulse which excites an electron on an individual complex. In the current issue of Journal of Chemical Physics 138, 144504 (2013) (free download), Benjamin Freyer, Flavio Zamponi, Vincent Juve, Johannes Stingl, Michael Woerner, Thomas Elsaesser and Majed Chergui report the first in-situ x-ray imaging of electron and atom motions induced by such an electron transfer excitation. For the prototype material [Fe(bpy)3]2+(PF6-)2, they show time-dependent 'electron maps' derived from x-ray snapshots taken with 100 fs long hard x-ray flashes. Taking x-ray snapshots at various times during and after the optical pulse that triggers the charge transfer, creates a molecular movie of electron and atom motions.
To the big surprise of the researchers, the time-dependent 'electron maps' reveal a transfer of electronic charge not only from the Fe atoms to the bipyridine units but - so far unknown -Â an even larger amount of electronic charge from the PF6- counterions to the bipyridine units. The analysis of the x-ray snapshots shows that the charge transfer affects approximately 30 complexes around the directly photo-excited one. This collective electron response is caused by the electric Coulomb forces between the different ions and minimizes the total electrostatic energy in the crystal. Such behavior is highly favorable for charge collection and injection in optoelectronic devices.
Figures and movie: Upper panels: sticks and balls model of the transition metal complex iron(II)-tris-bipyridine [Fe(bpy)3]2+. Iron-atoms (Fe) are brown, nitrogen (N) blue, carbon grey, and hydrogen (H) white. The six nitrogen atoms are at the corners of an octahedron around the Fe atom. The planes of the 3 bipyridine subunits (N2C10H8) are mutually perpendicular. Left lower panel: the counterions in our crystal are two hexa-fluoro-phosphate (PF6-) molecular subunits [phosphorus (P), fluorine (F)]. Again, the six F atoms are at the corners of an octahedron around the P atom. We show here a 3-dimensional surface of constant electron density ρ(r,t) = ρC = const. ρC was chosen in such a way that we are most sensitive to the motion of electronic charge located at the PF6- anion. In the attached movie we observe upon photo excitation a pronounced reduction of the electron density on that PF6- anion, i.e. a shrinkage of the iso-electron density surface. Right lower panel: 3-dimensional surface of constant electron density of the unit cell showing the spatial arrangement of Fe atoms (balls), bipyridine subunits (pretzel-like objects), and PF6- anions (octahedron-like stars).
Bottom: cartoon of the collective charge transfer in [Fe(bpy)3]2+(PF6-)2 which affects approximately 30 complexes around the directly photo-excited one. Blue: reduction of electron density, red increase of electron density.
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